Magnetically variable microstrip directional coupler deposited on ferrite substrate

ABSTRACT

Described is a novel variable coupling, directional coupler employing parallel microstrip transmission lines deposited on a substrate of ferrite material. Variable coupling is achieved by varying a magnetic field applied to the ferrite material normal to the direction of wave propagation along the microstrip transmission lines.

O Unlted States Patent 1111 3,585,531

[72] Inventors James E. Degenlord [56] References Cited Severua Park;UNITED STATES PATENTS A I N 2 gg 3,339,158 8/1967 Passaro 333/1.13,448,409 6/1969 Moose etaL. 333/84 [221 PM 3 448 410 6/1969 Parks333/24 1 Patemed June 15, I97 [73] Assignee Westinghouse ElectricCorporation OTHER REFERENCES Pitt b h, Jones & Whicker, Now-FerriteMicrostrip Devices,"

microwaves Jan. 1969, pp. 32 40, 333-24.l s41 MAGNETICALLY VARIABLEMICROSTRIP s' f'f' 'Z"'.' 1"???" lsaa'bach DIRECTIONAL COUPLER DEPOSITED0N i 11 g? z M L f FERRITE SUBSTRATE orneyrenson, 1p e an iegree e IClaim, 6 Drawing Figs.

[52] 1.8. CI. 333/Ll, ABSTRACT: Described is a novel variable coupling,333/10 directional coupler employing parallel microstrip transmission[5|] lnt.Cl H0lp 1/32, lines deposited on a substrate of ferritematerial. Variable HOlp 5/14 coupling is achieved by varying a magneticfield applied to the [50] Field of Search ..333/ 1.], l0, ferritematerial normal to the direction of wave propagation 24.1, 24.2, 84 Malong the microstrip transmission lines.

PATENTED JUN! 5197! SHEET 1 OF 2 Term/nation INVENTORS. JAMES E.DEGE/VFORD a MAGNETICALLY VARIABLE MICROSTRIP DIRECTIONAL COUPLERDEPOSITED ON FERRITE SUBSTRATE CROSS-REFERENCES TO RELATED APPLICATIONSU.S. application, Ser. No 809,669, filed Mar. 24, 1969, by Herbert W.Cooper and Robert O. Maclay, entitled Variable Coupling, MicrostripParallel-Line Directional Coupler," and assigned to the assignee of thepresent application BACKGROUND OF THE INVENTION With the availability ofmicrowave transistors and other semiconductor devices usable atmicrowave frequencies, the microstrip transmission line has found wideapplication because of its compatibility with the fabrication andinstallation of passive components and active devices on the samesubstrate with the transmission line. Essentially, a microstriptransmission line consists of a strip of conductive material,corresponding-to the center conductor of a coaxial transmission line,deposited on one side of a dielectric or semiconductive substrate byphotoresist techniques. The opposite side of the substrate is coveredwith a layer of conductive material comprising a ground plane andcorresponding to the outer cylindrical conductor of a coaxialtransmission line. With this configuration, and assuming that a sourceof wave energy is applied across the strip and ground plane on oppositesides of the substrate, and electric field is established between thetwo.

While parallel-line couplers utilizing microstrip circuitry have beendevised in the past, most of these are limited in the degree ofelectromagnetic energy coupling obtainable. That is, most prior artcouplers of this type require that any improvement in the degree ofcoupling between the branch lines be obtained by decreasing theperpendicular distance between the two microstrips, or requiredielectric overlays to increase the coupling. Spacings on the order ofabout 0.000l inch are required for a 3 db. coupling coefficient As willbe appreciated, this is very difficult to accomplish repeatedly withpresent photoresist techniques.

In the copending application, Ser. No. 809,669, a microstripparallel-line coupler is described which eliminates the necessity forextremely close spacing between microstrip transmission lines byinsertion of a PN junction in a semiconductive substrate between thetwo. By applying a bias across the PN junction and by varying that bias(forward up to the contact potential and reverse to breakdown), thedepletion-layer capacitance of the junction can be varied as well as thetotal coupling capacitance between the parallel microstrip transmissionlines.

While microstrip parallel-line directional couplers utilizing a PNjunction between the two to obtain a variable coupling effect are verysatisfactory for their intended purpose, they do have high insertionlosses. These are believed to be due primarily to a lowering of thesemiconductive substrate resistivity during processing to obtain the PNjunction.

SUMMARY OF THE INVENTION As an overall object, the present inventionseeks to provide a microstrip parallel-line coupler in which thecoupling effect can be varied by means of an applied magnetic field, andwherein the coupler is capable of achieving a high degree of couplingbetween the branch lines without resorting to extreme close spacingbetween branch lines.

Another object of the invention is to provide a microstrip parallel-linecoupler wherein the coupling coefficient between branch lines can bevaried, within limits, without changing the geometry of the microstripbranch lines.

Still another object of the invention is to provide a variable couplingmicrostrip parallel-line couple formed on a ferrite substrate on whichother circuit elements such as circulators and isolators can be formed.

In accordance with the invention, microstrip transmission lines aredeposited, by photoresist etching techniques, in parallel side-by-siderelationship on a ferrite substrate. The microstrip transmission linesextend parallel to each other through a distance equal to a quarterwavelength of the wave energy to be coupled. By applying a magneticfield across the ferrite in the region of the coupler and normal to thedirection of wave propagation through the coupler, the effectivepermeability of the ferrite can be made to vary with variations inapplied field. This change in permeability, in turn, is used to changethe propagation constant and even mode impedance of the transmissionlines deposited on the ferrite, thereby resulting in a variation in thecoupling coefficient of the coupler.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings which form a part of this specificationand in which:

FIG. 1 is a perspective view of the magnetically variable parallel-linedirectional coupler of the invention;

FIG. 2 is a top view of the appearance of the microstrip parallel-linedirectional coupler of the invention showing the manner in which it canbe connected to coaxial transmission lines;

FIG. 3 is a cross-sectional view taken trough the coupler of FIG. 1showing the manner in which wave energy is coupled from one parallelmicrostrip transmission line of the coupler to the other;

FIG. 4 illustrates the magnetic fields associated with the electricfields of FIG. 3;

FIG. 5 is a plot of coupling versus frequency for a parallel directionalcoupler fabricated in accordance with the teachings of the invention;and

FIG. 6 is a plot of insertion loss versus frequency for a couplerfabricated in accordance with the teachings of the inventron.

With reference now to the drawings, and particularly to FIGS. 1, 3, and4, the microstrip parallel-line directional coupler of the inventioncomprises a wafer 10 of ferrite material having its lower surfacecovered with a layer of metal 12 comprising a ground plane. The layer 12may typically comprise gold; and the ferrite substrate typicallyconsists of one of a number of different compounds, such as iron, zinc,manganese, magnesium, cobalt, aluminum and nickel-oxides. The ferritesare usually manufactured by pressing into shape the required mixture offinely divided metallic oxide powders and then firing the shaped mixtureat an elevated temperature. The product is a ceramic with a highelectrical resistance.

Deposited on the surface of the ferrite wafer I0, by conventionalphotoresist etching techniques, are parallel strip conductors l4 and 16which may, for example, have a width of about 0.010 inch and a lengthequal to a quarter wavelength of the wave energy which is to be coupled.Opposite ends of the two parallel strips 14 and 16 may be connected asshown in FIG. 2 to the center conductors of couplers 18 adapted forconnection to coaxial wave transmission lines. The outer cylindricalconductors of the transmission lines are threaded onto the couplers I8and are connected to the lower gold layer 12 comprising a ground plane.As a specific example, the wafer 10 may be 1 inch square and have athickness of about 0.025 inch.

On opposite sides of the wafer 10, in the region of the two parallelmicrostrip transmission lines 14 and 16 forming the coupler, areopposite ends ofa C-core 20 shown in FIG. I and provided with anencircling coil 22 adapted for connection to a variable source ofadirect current, not shown. This source of direct current can, therefore,produce a varying direct current field, H,,,., which passes through thearea of the parallel-line directional coupler comprising transmissionlines 14 and 16 in a direction normal to the direction of wavepropagation through the coupler.

The electric field lines of the wave energy pausing through the couplerare shown in FIG. 3. It can be seen that most ofthe field lines passbetween ground plane 12 and an associated one of the parallel microstriptransmission lines 14 and 16. However, assuming that wave energy iscoupled into one end of the strip 16, for example, a portion of thatwave energy will be coupled over to the other transmission line 14.

In FIG. 2, it can be seen that wave energy coupled into one end of thestrip 16 will be divided between the right and left ports 18, the upperport 18 comprising a termination for one end of the strip 14. Magneticfields produced by the wave energy circulate around the strips 14 and 16as shown in FIG. 4 and are, for the most part, perpendicular to theapplied magnetic field, H The wave energy, therefore, travels in aquasi- TEM mode wherein both the electric and magnetic vectors areperpendicular to the direction of wave propagationqThe voltage couplingcoefficient, K, of the coupler is equal to:

oe oo I K 2.0%..

where:

Z equals the even mode impedance of the coupled lines; and

Z equals the odd mode impedance of the coupled line.

It will be noted from FIG. 4 that the applied magnetic field, H willinteract with the horizontal component of the radio frequency magneticfields surrounding the microstrip transmission lines 14 and 16. Thisinteraction produces a change in the effective permeability of theferrite substrate 10 which, in turn, varies the quantities z 2 and K,the coupling coefficient, in the foregoing equation.

FIGS. and 6 are plots of coupling in db. versus frequency and insertionloss in db. versus frequency for a coupler fabricated in accordance withthe teachings of the invention. The coupler was fabricated on a 0.025inch thick ferrite substrate and designed for a nominal coupling of db.with no applied field at the center frequency of about 9.5 gigahertz,The upper dotted lines in FIGS. 5 and 6 illustrate the coupling andinsertion loss, respectively, for an applied magnetic field of +2kilogausses. The center, solid lines illustrate the coupling andinsertion loss, respectively with no applied external magnetic field Hand the lower broken lines illustrate the coupling and insertion loss,respectively, with the magnetic field reversed and at an intensity of 2kilogausses. It can be seen from FIGS. 5 and 6 that the coupling andinsertion losses remain essentially constant over the frequency range of9 to 10 gigahertz. Furthermore, as the magnetic field is decreased fromits positive maximum through zero and the increased in the negativedirection, the coupling increases while the insertion loss decreases.

Although the invention has been shown in connection with a certainspecific embodiment, it will be readily apparent to those skilled in theart that various changes in form and arrangement of parts may be made tosuit requirements without departing from the spirit and scope of theinvention.

We claim as our invention:

1. A parallel-line directional coupler comprising a substrate of ferritematerial having deposited on one surface thereof parallel microstriptransmission lines, the microstrip transmission lines extending parallelto each other for a distance equal to a quarter wavelength of the energyto be coupled, conductive material deposited on the other side of thesubstrate and forming a ground plane, means for applying a magneticfield across said ferrite material in a direction normal to thedirection of the wave propagation through said microstrip transmissionlines and through said ferrite substrate in the area of said parallelmicrostrip transmission lines whereby the effective permeability of theferrite material is determined by the magnetic field as well as thedegree of coupling between the parallel transmission lines, and meansfor varying said magnetic field to vary the coupling coefficient betweenthe parallel microstrip transmission lines.

1. A parallel-line directional coupler comprising a substrate of ferrite material having deposited on one surface thereof parallel microstrip transmission lines, the microstrip transmission lines extending parallel to each other for a distance equal to a quarter wavelength of the energy to be coupled, conductive material deposited on the other side of the substrate and forming a ground plane, means for applying a magnetic field across said ferrite material in a direction normal to the direction of the wave propagation through said microstrip transmission lines and through said ferrite substrate in the area of said parallel microstrip transmission lines whereby the effective permeability of the ferrite material is determined by the magnetic field as well as the degree of coupling between the parallel transmission lines, and means for varying said magnetic field to vary the coupling coefficient between the parallel microstrip transmission lines. 